The invention is directed to the use of random dot stereograms as stereopsis test patterns for adjustment of commercially available stereomicroscopes for inspection of three-dimensional objects. The stereopsis test patterns are an objective standard for adjusting a stereomicroscope, a means for training inspectors in the use of the microscope, a test for screening personnel for the presence of stereopsis, and can be used to monitor inspection rate and fatigue during the inspection period.
Many products depend on visual inspection of their surface for quality control. For example, microscopy is useful in the microelectronics industry for device fabrication and inspection. Generally, binocular microscopes are employed for this purpose. The binocular microscope has two eyepieces, but presents the same planar image to each eye. In contrast, the stereomicroscope has two optical viewing paths which act as extensions of the eyes so that each receives a separate view of the image with the appropriate binocular parallax to give a three-dimensional effect. A stereomicroscope is used to examine three-dimensional objects or three-dimensional defects such as nodules or pits which are difficult to distinguish with a binocular microscope. However, there are two major disadvantages to the use of a stereomicroscope. First, a portion of the population is unable to attain stereopsis and gains little benefit from the use of a stereomicroscope. In fact, two percent of the total population is stereoblind and fifteen percent of the population is stereodeficient. (Julesz, Foundations of Cyclopean Perception, University of Chicago Press, Chicage, 1971, page 270). These people are unaware of their deficiency because of the large number of monocular depth cues, such as movement parallax and familiarity, which conceal the absence of stereopsis.
Random dot stereograms are devoid of monocular depth cues and can be used as a means to detect the absence of stereopsis. Random dot stereograms (Julesz, The Bell System Technical Journal, Vol. 39, No. 5, 1960, pages 1125-1162) are computer generated patterns which contain no depth cues other than binocular parallax (a horizontal shift) between the left and right images. When these images are stereoptically fused, predetermined patterns printed on them will appear in depth. The stereograms were originally developed for psychological experiments tracing the flow of perceptual information in the central nervous system. Besides detection of stereopsis, opthalmologists and optometrists have also used stereograms to test pattern recognition in children. A few medical schools have adopted their use in screening for admission to the specialty of X-ray fluroscopy in which heart catheterizations are done with stereo X-ray viewing.
A second disadvantage in the use of a stereomicroscope is the difficulty of adjusting the microscope to obtain stereopsis. Adjustment of a stereomicroscope is a subjective procedure involving focussing and stereoalignment. Focusing refers to adjustment of the field of depth and stereoalignment refers to adjustment of the resolution and interocular distance. The trouble in adjustment arises because there are no objective standards on which to base judgment. Pictures of a sample viewed through a properly focused microscope are often available for comparative purposes. Such pictures are employed in training microscope technique, uually in conjunction with an explanation of the mechanics of operation. Training microscopes are also available which have two sets of eyepieces so that the instructor can observe what a student is doing. However, criticism by the instructor depends on his own judgment.
Focal length and resolution may be determined or adjusted by means of commercial test patterns. However, the test patterns commonly used, such as Abbe or Ealing patterns, consist of a series of equally spaced high contrast lines which decrease in size. The resolution or lateral area is determined by that fineness of lines where the lines just turn to grey. The same standard is used to determine the focal length or field of depth.